AAPG Europe Regional Conference, Global Analogues of the Atlantic Margin

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Structural restorations in an unusual passive margin: Alentejo Basin, SW Portugal

Abstract

The SW Iberian margin is not a “quiet” passive margin. Its peculiar position at the limit of three rift areas and nearby a plate boundary, coupled with Iberia’s history of plate rotation, is responsible for a prolonged history of deformation as unraveled by a number of positive structures still active at present day. Detached by the other basins of the Portuguese margin by a huge NE-SW strike slip fault system, the Alentejo basin underwent extension in the form of two main rifting episodes punctuated by sag phases. At the same time, transtension was acting in the basin due to the interference of the Tethyan rifting from East. This is clearly recognizable in the en échelon arrangement of the main depocentral areas, elongated and narrow subsiding pull apart basins, NE-SW directed. From Upper Cretaceous onward, transpression took place due to the two convergences of Iberia with Eurasia first and Africa later. The compression inverted the main depocentral areas and reactivated in reverse or strike-slip motion the basin-bounding faults. This transpressional phase is responsible for the high number of structural-stratigraphic potential traps which in the basin are widespread and gives rise to huge positive structures on the hyperextended domain. Decoupling of the deformation, particularly evident in the southern area, points to the presence of a ductile unit in the lower part of the Jurassic section that acts as a detachment level for the Tertiary inversion structures. Restoration on the central part of the basin gives a relative timing of the structuration with possible very young traps supposedly completely formed (with a seal and a proper burial depth) at the end of Tertiary. Additional restored lines helped fixing the interpretation and provided additional hints on the structural evolution of the basin, showing transpression pulses at the end of Jurassic. In the southern area, the restoration allowed to improve the understanding of the depth structural framework, suggesting an alternative interpretation of the main fault pattern in the deep sequences of the Marquis de Pombal structure.